Flexible sheathing tube construction, and method for fabrication thereof

ABSTRACT

A flexible sheathing tube suitable for encasing an elongated flexible body like a flexible insertion rod of an endoscope or other insertion type examination instrument, and a method for fabrication of such sheathing tubes. The flexible sheathing tube being of the type is basically provided with a flexible base structure having at least an open helical coil member of a cylindrical shape, a protective mesh sleeve fitted on the helical coil member, and an insulating outer coating layer formed on the protective mesh sleeve. The sheathing tube is further provided with an inner coat layer of a resilient material formed on the inner side of the helical coil member in such a manner as to cover the inner periphery of the helical coil member completely, filling in gap spaces between open helices of the coil member.

This is a Division, of application Ser. No. 08/583,162 filed on Jan. 4,1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of The Art

This invention relates to a flexible sheathing tube constructionparticularly suitable for use on insertion type examination instrumentshaving a flexible elongated component part fitted in a flexiblesheathing tube as in the case of flexible insertion rods of endoscopesand ultrasound probes which are designed to be introduced into internalcavities in humane bodies or machines or other internal spaces, and amethod for fabrication of such flexible sheathing tube.

2. Prior Art

For instance, an endoscope, which is typical of instruments designed tobe introduced into internal cavities or spaces in human bodies ormachines for examination purposes, is largely constituted by amanipulating head assembly providing manipulating control means for theendoscope, an elongated flexible insertion rod containing a componentpart or parts for carrying out examinations in intracavitary portions ofhuman body and extended forwardly from the manipulating head assembly,and a flexible universal cable extended rearward from the manipulatinghead assembly to connect the endoscope detachably to an illuminationlight source or to a signal processing unit in case of an electronicendoscope. Except for a proximal end portion which is connected to themanipulating head assembly and a short rigid tip end section whichaccommodates endoscopic observation means at the distal end of theendoscope, the endoscopic insertion rod is constituted by a tubularflexible rod section almost over its entire length, the flexible rodsection being capable of flexibly bending its body into conformity withthe shape of a path of insertion. Normally, the flexible rod section isconnected to the rigid tip end section through an angle section whichcan be manipulated through an angle knob provided on the manipulatinghead assembly to turn the rigid tip end section into a desireddirection.

In order to illuminate dark intracavitary portions under endoscopicobservation, the rigid tip end section is normally provided with anillumination window in addition to an observation window, permitting theoperator to observe intracavitary regions of particular interest underilluminated conditions. Accordingly, it is essential for an endoscopicobservation means to have an illumination system in combination with anendoscopic observation system.

Typically, an endoscopic illumination system is constituted by a lightguide in the form of a bundle of extra fine fiber optics, and anillumination window located in front of an light emitting end of thelight guide to disperse illumination light rays over a predeterminedrange within an intracavitary region. The light guide is extendedthrough the entire length of the insertion rod and down to the proximalend of the universal cable via the manipulating head assembly.

On the other hand, an endoscopic observation system includes anobjective lens which is fitted in the observation window, and an imagepickup means which is located at the focus of the objective lens, i.e.,an electronic image sensor like a CCD or a light entrance end of anoptical image guide in the form of a fiber optics bundle or the like. Incase of an electronic endoscope, a signal cable from a CCD is alsopassed through the flexible insertion rod and the universal cable viathe manipulating head assembly together with the afore-mentionedillumination light guide. In case of an optical endoscope, an imageguide is passed through the flexible insertion rod up to an eyepiecewhich is connected to the housing of the manipulating head assembly ofthe endoscope.

In addition to the above-described endoscopic illumination andobservation systems which are minimum essentials, endoscopes aregenerally provided with means for bioptic or therapeutic treatments suchas sampling of cells, extraction of diseased portions, stanching etc. Inorder to permit insertion of forceps, high frequency surgicalinstruments or other instruments which are necessary for thesetreatments, endoscopes are provided with the so-called biopsy channelextending from the manipulating head assembly down to the distal end ofthe flexible insertion rod. Besides, it is often the case that theendoscopic insertion rod includes a cleansing means for cleaning theobservation window which is susceptible to contaminations with bodyfluids. An observation window cleansing means of this sort usuallyincludes an air/water feed nozzle which is located and opened in theproximity of the observation window at the distal end of the insertionrod. The air/water feed nozzle is connected to an air/water feed tubewhich is placed in the rigid tip end section and the angle section ofthe endoscopic insertion rod. In the flexible rod section, the air/watertube is bifurcated into an air feed tube and a water feed tube which areextended all through the flexible rod section up to air and water supplyports on the manipulating head assembly of the endoscope.

Accordingly, the flexible rod section of the endoscopic insertion rodneeds to receive therein the above-described light guide, signal cable(or image guide) which are extended to or from the endoscopicobservation means on the rigid tip end section, and in some cases needto further receive the biopsy channel, air feed tube and water feed tubeas mentioned above. As explained hereinbefore, the flexible rod sectionshould be capable of bending its body in arbitrary directions inconformity with the shapes of various bends in a path of insertion.Therefore, the light guide or other elongated members to be fitted inthe flexible rod section need to be formed of a flexible or pliablematerial.

Further, the flexible rod section which contains fragile component partlike a light guide is required to have satisfactory properties in shaperetainability and anti-crushing strength, together with unresistingbending flexibility for advancement along a path of insertion whichcould take various shapes. In other words, despite the requirement foreasily bending pliability, the flexible rod section is at the same timerequired to have a structure which is rigid enough for transmitting apropelling thrust securely to the rigid tip end section at the time ofinsertion into an intracorporeal portion to be examined.

In order to satisfy these requirements, the flexible rod section of anendoscope is usually composed of a flexible tubular base structure inthe form of an open helical coil structure which is formed by winding anarrow thin resilient metal strip helically into a cylindrical shape ina predetermined open pitch, a protective mesh sleeve of wire nettingwhich is fitted on the helical substructure, and an outer skin layer ofa resilient synthetic resin material laminated to cover the protectivenet. Usually, the flexible base structure of the sheathing tube has adouble coil construction consisting of inner and outer open helical coilmembers of opposite winding directions. The protective mesh sleeve isimpregnated with an adhesive and thereby securely bonded to therespective open helical coil members of the flexible base structure. Theflexible sheathing tube, which is composed of, from the inner side, aflexible base structure of the double coil construction, a protectivemesh sleeve and an outer skin layer in the above-described manner, canperform the functions of flexibly bending an insertion rod in desireddirections and retaining the shape of the rod, particularly, the shapeof the internal space of the insertion rod against deformations.

However, in addition to the above-described requirements, the flexiblesheathing tube of the endoscopic insertion rod has to meet otherrequirements, for example, smooth operationability at the time ofinsertion into the body of a patient and reductions in diameter forlessening pains on the part of patient. In this regard, if one tries toreduce the diameter of the flexible sheathing tube while maintaining itsshape retainability and anti-crushing strength, there will arises aproblem of frictional contact of internally fitted endoscopic componentparts such as the light guide and biopsy channel, with each other orwith inner surfaces of the flexible sheathing tube itself. Accordingly,it becomes necessary to provide measures for protecting these internallyfitted components against damages which might result from suchfrictional contact, particularly, from frictional contact with thehelical metal coil members on the innermost position of the flexiblesheathing tube. This is because, even if the metal strips of the openhelical coil members were rounded off beforehand at the respectivelateral side edges, they would still have possibilities of damaging orbreaking a fragile component part like the light guide easily whenforced into frictional sliding contact with each other. Besides,considering the use of a high frequency surgical instrument through thebiopsy channel, preferably any metal member should not be allowed toremain in an exposed state on the inner side of the flexible sheathingtube.

Further, since the inner and outer metal coil members of theabove-mentioned flexible base structure of the sheathing tube are in theform of open helical coils each having the respective helices spacedfrom each other by a gap space of a predetermined width and placed inposition in an overlapped state in surface contact with helices of othercoil member, it is very likely for the inner and outer helical coilmembers to be subjected to a twisting force in different degrees and indifferent directions when a bending force is exerted on the insertionrod, resulting in relative sliding contact of the two helical coilmembers and in irregular spacings between the individual helices of therespective coil members. As soon as the insertion rod is stretched againinto a rectilinear form, normally such irregularly spaced helicesrestore original regularly spaced conditions by resiliency of the coils.However, if the flexible insertion rod section is bent repeatedly toexert deformative forces on the flexible sheathing tube at a relativelyhigh frequency, the individual helices of the inner and outer coilmembers tend to remain in irregularly spaced positions, giving rise toirregularities in rigidity in the axial direction of the endoscopicinsertion rod.

From a standpoint of higher adaptability of the insertion rod to a pathof insertion, the sheathing tube is preferred to have as highflexibility as possible. However, the higher the flexibility of thesheathing tube, the lower becomes the strength in stretching andcontracting directions of the insertion rod. Accordingly, at the time ofinsertion of the flexible insertion rod, there might occur a situationin which a fore end portion of the flexible sheathing tube is compressedtoo easily, narrowing the spacings between the helices in that portionto such a degree as to lower the flexibility of the insertion rodconspicuously in bending directions. Besides, application of acompressive force on the flexible sheathing tube could cause detachmentof the protective mesh sleeve off the outer helical coil member, whichin turn could lead to a detrimental damage to the outer skin layer ofthe sheathing tube. Namely, a flexible sheathing tube which has an outerand inner helical coil members simply superposed one on the other cannotbe considered to be sufficient in durability and reliability.

Particularly, an axial fore end portion of the endoscopic insertion rodis required to have a high degree of flexibility to ensure its easybending motions along a path of insertion. On the other hand, on theside of the proximal end, the insertion rod is required to have acertain degree of rigidity to improve transmission of a propelling forceor thrust through the rod being advanced toward an intracavitary portionof interest. As for means for controlling the rigidity in the axialdirection of the flexible sheathing tube, attempts have been made tovary the width of metal strips for the helical coil members or of gapspaces between the respective helices of the coil members which arewound in an open pitch, to vary the mesh size of the protective meshsleeve, or to vary the amount of application of an adhesive in the axialdirection of the sheathing tube. Since all of these means have inherentproblems, there have been strong demands for improved means which canvary the rigidity in the axial direction of the flexible sheathing tubein a stable and reliable manner.

SUMMARY OF THE INVENTION

With the foregoing situations in view, it is an object of the presentinvention to provide a flexible sheathing tube construction suitable forencasing an elongated flexible body securely in a protected state, andparticularly suitable for use on a flexible insertion rod of anendoscope or other insertion type examination instruments, the flexiblesheathing tube having controlled flexibility in the axial directionthereof to ensure improved controllability and maneuverability of aninsertion rod or the like at the time of introduction into an internalcavity to be examined.

It is another object of the present invention to provide a flexiblesheathing tube of the sort as mentioned above, which is suitable forsheathing an elongated fragile component part such as endoscopic lightguide or the like securely in a protected state.

It is still another object of the invention to provide a flexiblesheathing tube particularly suitable for use on an endoscopic flexibleinsertion rod containing a biopsy channel for insertion of a highfrequency surgical instrument or the like, the flexible sheathing tubebeing interiorly provided with an electrical insulation keeping metallichelical coil members of a flexible base structure of the sheathing tubesecurely out of electrical contact with such a high frequencyinstrument.

It is a further object of the present invention to provide a flexiblesheathing tube for an endoscopic insertion rod, which is arranged tosuppress frictional sliding contact of inner and outer open helical coilmembers of a flexible base structure of the sheathing tube even in theevent a compressive force is applied on the sheathing tube, therebypreventing open helices of the respective coil members from being spacedirregularly to such a degree as to cause conspicuous irregularvariations in rigidity in the axial direction of the endoscopicinsertion rod.

It is another object of the invention to provide a flexible sheathingtube which can prevent detachment of a protective mesh sleeve from a ofhelical coil member of a flexible base structure of the sheathing tube,precluding damages as would be caused to an outer skin layer bydetachment of the protective mesh sleeve.

It is still another object of the invention to provide a flexiblesheathing tube of the sort as mentioned above, which is capable ofmaintaining predetermined variations in rigidity in a stabilized statein the axial direction of the tube.

In accordance with the present invention, the above-stated variousobjectives are achieved by the provision of a flexible sheathing tube ofthe sort composed of, from the inner side, a flexible base structureconsisting of at least an open helical coil member wound in acylindrical shape in a predetermined open pitch, a protective meshsleeve fitted on the helical coil member, and an insulating coatinglayer formed on the protective mesh sleeve, characterized in that theflexible sheathing tube is provided with an inner coat layer of aresilient material formed on the inner side of the helical coil memberin such a manner as to cover the inner periphery of the helical coilmember completely, filling in gap spaces between individual helices ofthe coil member. In this flexible sheathing tube construction, theflexible base structure may further include, in overlapping relationwith the first open helical coil member with open helices, a second openhelical coil member with open helices of the opposite winding directionrelative to the helices of the first helical coil member, and anintermediate mesh sleeve fitted between the first and second coilmembers.

In accordance with the present invention, there is also provided amethod for fabricating a flexible sheathing tube of the construction asdefined above, the method essentially including the steps of: fitting aprotective mesh sleeve on an open helical coil before or after formingan insulating outer coat layer thereon; pouring a liquidized resilientmaterial into the helical coil through one end thereof while closing theother end with a closure means; holding the liquidized resilientmaterial in the helical coil over a predetermined time length; removingthe closure means to let the liquidized resilient material drain out ofthe helical coil, leaving a deposition layer of the resilient materialon the inner periphery and between the helices of the open helical coil;and drying the deposition layer of the resilient material.

Further, in order to vary rigidity in the axial direction of theflexible sheathing tube by way of the resilient inner coat layer, thefabrication method may comprise the steps of: pouring a first liquidizedresilient material into the open helical coil through one end thereofwhile closing the other end with a closure means as described above;holding the first liquidized resilient material in the helical coil fora predetermined time length; putting the closure means on and off theother end of the coil member for a number of times to form a firstdeposition layer of the resilient material on the inner periphery andbetween open helices of the helical coil; pouring a second liquidizedresilient material of different property or density from the firstliquidized resilient material into the helical coil through the oppositeend thereof up to a parting line of the first deposition layer whilestopping the other end with a closure means; holding the secondliquidized resilient material in the helical coil for a predeterminedtime length; and removing the closure means to let the second liquidizedresilient material drain out of the helical coil, leaving a seconddeposition layer of different flexibility on the inner periphery andbetween open helices of the helical coil.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become apparent from the following particular description of theinvention, taken in conjunction with the accompanying drawings whichshow by way of example some preferred embodiments of the invention andin which:

FIG. 1 is a schematic view of a typical endoscope, showing the layout ofits major parts;

FIG. 2 is an enlarged fragmentary sectional view of a flexible sheathingtube in a first embodiment of the present invention;

FIG. 3 is a schematic illustration of a stage of forming a resilientcoat layer on the inner periphery of a flexible base structure of thesheathing tube of the first embodiment;

FIG. 4 is a schematic illustration of another stage of forming aresilient coat layer on the inner periphery of the flexible basestructure of the sheathing tube;

FIG. 5 is a view similar to FIG. 2 but showing a flexible sheathing tubeaccording to a second embodiment of the invention;

FIG. 6 is a schematic illustration of a stage of forming a resilientcoat layer on the inner periphery of a flexible base structure of thesheathing tube of the second embodiment;

FIG. 7 is a schematic illustration of another stage of forming aresilient coat layer on the inner periphery of the flexible basestructure of the sheathing tube of the second embodiment;

FIG. 8 is a schematic partly removed view of a flexible sheathing tubein a third embodiment of the invention;

FIG. 9 is a fragmentary sectional view, showing on an enlarged scalemajor components of the flexible sheathing tube shown in FIG. 8;

FIG. 10 is a fragmentary sectional view, showing on an enlarged scalemajor components of a flexible sheathing tube in a fourth embodiment ofthe invention;

FIG. 11 is a partly cutaway sectional view, showing major component of aflexible sheathing tube in a fifth embodiment of the invention;

FIG. 12 is a schematic view of an outer helical coil member, showing ajoint portion of broad and narrow helical coil strips; and

FIG. 13 is a schematic illustration of another joint arrangement forbroad and narrow helical coil strips.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, there is shown a layout of major componentsof an endoscope, which is typical of insertion type examinationinstruments. In this figure, indicated at 1 is a manipulating headassembly of the endoscope, at 2 is a flexible insertion rod to beintroduced into an intracavitary portion which needs endoscopicexamination or observation, and at 3 is a flexible universal cable whichconnects the endoscope to a light source and/or an electrical orultrasound signal processor depending upon the type of the endoscopeconcerned. In case the endoscope is of the sort which is arranged formedical use, a flexible rod section 2a extends almost over the entirelength of the insertion rod 2 from its proximal end which is connectedto the manipulating head assembly 1 to its fore end which issuccessively connected to an angle section 2b and a rigid tip endsection 2c. The flexible rod section 2a needs to be capable of bendingits body in arbitrary directions in conformity with the shape of a pathof insertion. Opened in the distal end face of the rigid tip end section2c are an illumination window and an observation window of endoscopicobservation means, along with an exit opening of a biopsy channel forinsertion of forceps or other bioptic or surgical instruments. The anglesection 2b can be bent in an arbitrary direction through manipulation ofan angle knob 4 on the manipulating head assembly 1 to turn the rigidtip end section 2c in a desired direction.

The endoscopic flexible insertion rod 2, which is designed to beinserted into dark intracavitary portions of patient, should be able toproject illumination light through the illumination window toward anintracavitary region under observation through the observation window.For this purpose, a flexible light guide is passed through the universalcable 3 and the insertion rod 2 to the rigid tip end section 2c via themanipulating head assembly 1. At the proximal end of the universal cable3, the light guide is disconnectibly connected to a light source.Illumination light from the light source is transmitted through thelight guide to its fore light emitting end which is disposed face toface with the illumination window on the distal end face of the rigidtip end section 2c. Further, in case of an electronic endoscope whichemploys an electronic image pickup means like CCD as endoscopicobservation means, a signal cable from an image pickup means which islocated face to face with the observation window is also passed throughthe flexible rod section 2a and the universal cable 3 along with thelight guide via the manipulating head assembly 1. At the proximal end ofthe universal cable 3, the signal cable from the electronic image pickupmeans is connected to a connector member, which is in turndisconnectibly connected to a video signal processor.

As mentioned hereinbefore, the flexible rod section 2a of the endoscopicinsertion rod 2 is required to be able to bend its body in arbitrarydirections in conformity with the shape of a path of insertion, andtherefore the above-described light guide, biopsy channel and signalcable are all fitted in a flexible sheathing tube 10 of the constructionas shown particularly in FIG. 2. In FIG. 2 and following figures, thecomponent parts which are fitted in the flexible sheathing tube 10 areomitted for the sake of simplicity of illustration.

As seen in FIG. 2, located in the innermost position of the flexiblesheathing tube 10 is a flexible base structure in the form of an openhelical coil member 11 which is formed by helically winding a narrowmetal strip in a predetermined open pitch. In this particularembodiment, the flexible base structure of the sheathing tube 10consists of a couple of open helical coil members which are fitted oneon the other in an overlapped state, namely, an inner open helical coilmember 11a and an outer open helical coil member 11b having open helicesof opposite winding directions. A mesh sleeve 12 of fine metal wirenetting is fitted on the double coil base structure 11, i.e., on theopen helical coils 11a and 11b, to form a protective mesh layertherearound. The protective mesh sleeve 12 is impregnated with anadhesive and thereby securely bonded to the open helical coil members11. Laminated on the protective mesh sleeve 12 is an outer skin layer 13of a resilient material such as urethane resin or the like.

Indicated at 14 is an inner coat layer of an electrically insulatingresilient material such as natural rubber, urethane rubber or the like.This inner coat layer 14 is formed in such a way as to cover completelythe inner periphery of the inner coil member 11a of the double coilstructure 11, more specifically, to cover the inner side of the innercoil member 11a and at the same time fill in gap spaces between theindividual helices of the inner and outer open coil members 11a and 11bby way of filler portions 14a and 14b. Accordingly, the individualhelices of the inner and outer open coil members 11a and 11b are more orless embedded in the resident material of the inner coat layer 14 whichfills the gap spaces between the respective helices.

In this instance, the inner coat layer 14 can be formed over the entirelength of the flexible sheathing tube 10 either uniformly in terms ofits thickness and resiliency or with a predetermined gradation orvariations in flexibility or rigidity in the axial directions forexample, to provide an endoscopic insertion rod which has higherrigidity at its proximal end than in its fore end portion which isconnected to the angle section 2b of the flexible insertion rod 2. Inthis regard, the degree of flexibility or rigidity of the inner coatlayer can be varied by changing its properties or thickness axiallyalong the elongated body of the flexible sheathing tube, for example, inorder to impart a higher degree of flexibility to a fore end portion ofan endoscopic insertion rod to be inserted into intracavitary regions ofpatient while imparting a higher degree of rigidity to a rear endportion which is always located outside during endoscopic examination.

The above-described inner coat layer 14 can be formed by a method asillustrated in FIGS. 3 and 4.

In the first place, the protective mesh sleeve 12 is fitted on theflexible double coil structure 11, and an adhesive agent is impregnatedinto the protective mesh sleeve 12 for secure bondage to the double coilstructure 11. An outer skin layer 13 is then laminated on the protectivemesh sleeve 12 to form a sheathing tube pre-assembly 20 for the flexiblesheathing tube 10 as shown in FIG. 3. Thereafter, a closure means 21 isfitted on one end of the sheathing tube pre-assembly 20, namely, at oneend which will be form a proximal end of an endoscopic insertion rod incase the sheathing tube is to be applied to an endoscope. Morespecifically, the closure means 21 is tightly and securely fitted on oneend of the sheathing tube pre-assembly 20. A rubber solution 23,prepared by dissolving rubber in a solvent, is then filled in thesheathing tube pre-assembly 20 via feed pipe 22 to deposit an inner coatlayer 14 interiorly of the pre-assembly 20.

While the rubber solution is retained within the sheathing tubepre-assembly 20 for a predetermined time length, rubber deposits on theinner surfaces of the inner helical coil member 11a of the flexible coilstructure 11 and at the same time fills in the gap spaces between theindividual helices of the inner and outer helical coil members 11a and11b. When the closure means 21 at one end of the sheathing tubepre-assembly 20 is removed as shown in FIG. 4, excess rubber solution isdischarged therethrough, leaving a deposition rubber layer of apredetermined thickness on the inner periphery of the flexible sheathingtube pre-assembly 20. Upon drying out the deposition rubber layer, itcures into a form which can serve as the above-described inner coatlayer 14. While the deposited rubber layer is being dried, the rubberdeposit which is still in a liquidized state tends to flow gradually inthe downward direction. Therefore, by slowing down the drying speed, thethickness of the inner coat layer 14 can be varied gradually in theaxial direction from the top upper to bottom end of the sheathing tube10.

Shown in FIG. 5 is a flexible sheathing tube construction with an innercoat layer which has different characteristics or properties on frontand rear parts of the sheathing tube 10. In this case, the flexibilityof the inner coat layer is varied by forming a first inner coat layer14R of low flexibility in a rear part of the sheathing tube pre-assembly20 and a second inner coat layer 15F of higher flexibility in a frontpart. FIGS. 6 and 7 show a method for forming such an inner coat layerwith different properties in flexibility at the opposite ends thereof.Firstly, as shown in FIG. 6, a first rubber solution 23a which, issupplied through a first feed pipe 22a, is poured into a sheathing tubepre-assembly 20 which is closed at its lower end by a closure means 21,filling the first rubber solution 23a up to a predetermined level L inthe axial direction of the sheathing tube pre-assembly 20. Upon lapse ofa predetermined time length, the closure means 21 is removed from thelower end of the assembly 20 to drain excess rubber solution 23atherethrough, followed by drying of a deposition rubber layer whichremains in the sheathing tube pre-assembly 20, to form one inner coatlayer (e.g., the first inner coat layer 14R) on part of the innerperiphery of the sheathing tube assembly 20. Thereafter, the sheathingtube pre-assembly 20 is turned upside down, and the closure means 21 isfitted on the other end of the assembly 20 which is now on the lowerside. In a next stage, as shown in FIG. 7, a second rubber solution 23b,which is supplied through a feed pipe 22b, is filled in the sheathingtube pre-assembly 20 until the second rubber solution 23b reaches alevel of a parting line or a border line of the first inner coat layer14R formed by the first rubber solution 23a. After holding the secondrubber solution 23b in the sheathing tube pre-assembly 20 over apredetermined time length, the closure means 21 is removed from thelower end of the sheathing tube pre-assembly 20 to drain excess secondrubber solution therethrough. A deposition rubber layer which remains inthe sheathing tube pre-assembly 20 is then dried to form another innercoat layer of different properties (e.g., the second inner coat layer14F) on the inner periphery of the sheathing tube pre-assembly 20 on theother side of the above-mentioned parting line. In case the flexiblesheathing tube 10 is to be ultimately assembled into an endoscopicinsertion rod, it is preferred to employ a quick-drying type rubbersolution especially for the second inner coat layer 14F which is to belocated on the front side of the insertion rod where it is undesirablefor the rubber deposition layer to pick up thickness while drying, ascompared with the first inner coat layer 14R to be located on the rearside of the insertion rod.

In case the flexible sheathing tube 10 of the above-describedconstruction is fitted on a flexible rod section 2a of an endoscopicinsertion rod 2, the insertion rod 2 is met by resisting forces uponintroduction into the body of patient. Particularly in case of anendoscope which is designed for examination of large intestine, theintroduction of the insertion rod 2 is resisted by extremely largeforces. Therefore, in order to push in the insertion rod 2 straightforward, it becomes necessary to transmit a thrust or a propelling forceeffectively all the way to the fore tip end of the rod. On such anoccasion, despite the resiliency in physical properties, the resilientrubber material of the inner coat layer 14 which, by way of theafore-mentioned filler portions 14a and 14b, completely fills in the gapspaces between the respective open pitch helices of the inner and outercoil members 11a and 11b of the flexible base structure 11 which extendsthrough the entire length of the flexible rod section 2a, functions totransmit a thrust or a propelling force securely to the rigid tip endsection 2c at the distal end of the insertion rod 2. Accordingly, at thetime of insertion into an intracavitary portion, there are lesspossibilities of the insertion rod 2 being forcibly bent by a resistiveforce in its rear end portion outside the body of patient to such adegree as to make a further advancement of the rod impossible.

Of course, the filler portions 14a and 14b which fill in the gap spacesbetween the individual helices of the inner and outer coil members 11aand 11b are resilient enough for permitting smooth flexing movements ofthe insertion rod 2 along a path of insertion. A propelling thrust canbe transmitted more efficiently to the rigid tip end section 2c of theinsertion rod 2 especially in case the inner coat layer 14 of thesheathing tube 10 consists of two sections of different properties inflexibility, i.e., a first inner coat layer 14R of lower flexibilityformed on the rear side and a second inner coat layer 14F of higherflexibility formed on the front side of the flexible sheathing tube 10,for imparting higher flexibility to a fore end portion of the insertionrod which has to be bent in arbitrary directions along a path ofinsertion, while imparting higher rigidity to a rear end portion of theinsertion rod which is normally located outside the body of patient totransmit axial propelling forces toward the fore end of the insertionrod. When the insertion rod is bent, the filler portions 14a and 14b ofresilient material in a bent rod portion undergo elastic deformations,but they restore initial conditions as soon as the insertion rod isstraightened again, maintaining gap spaces of a predetermined widthbetween the respective helices of the inner and outer coil members 11aand 11b. Therefore, there are less possibilities of irregular variationsin rigidity developing in a particular region of the flexible rodsection 2a during use over an extended period of time.

When the insertion rod 2 is bent in conformity with the shape of a pathof insertion as mentioned hereinbefore, the elongated component partswhich are fitted in the insertion rod 2, including the light guide,biopsy channel and so forth, are caused to move in radially outward andinward directions as well as in axial directions in relation withflexures of the insertion rod. On such occasions, however, theinternally fitted component parts are caused to slide along the innersurface of the flexible sheathing tube 10, namely, in contact with thesurfaces of the resilient inner coat layer 14 which covers and holds theflexible coil structure 11 in an unexposed state to precludepossibilities of damages to the internally fitted component parts.

The rubber material which constitutes the inner coat layer 14 is notnecessarily of lubricative or anti-frictional nature by itself. That isto say, for the purpose of lessening frictions between the inner coatlayer 14 and internally fitted components of the insertion rod, theinner coat layer 14 may contain powdery anti-friction material such asmolybdenum disulfide, carbon or the like. Such powdery anti-frictionmaterial can be mixed into the rubber solution before forming the innercoat layer 14, or applied on wet surfaces of the inner coat layer 14before drying.

Alternatively, powdery anti-friction material may be filled in theflexible sheathing tube after formation of the inner coat layer 14 sothat it deposits on the surfaces of the internally fitted componentparts and the inner coat layer 14 to be brought into sliding contactwith each other. The inner coat layer 14 which is formed of a resilientmaterial can be easily treated by any one of these methods to bearpowdery anti-friction material securely on its surfaces. Besides, oncedeposited, the powdery anti-friction material tends to sink into thesurfaces of the resilient inner coat layer 14 without being shifted intoparticular localities by sliding movements of the internally fittedcomponent parts, thus ensuring their smooth sliding movements at anypoint within the length of the flexible sheathing tube 10.

Further, the metallic component parts of the flexible sheathing tube 10,that is, the flexible coil structure 11 and the protective wire meshsleeve 12 are sandwiched securely in an electrically insulated statebetween the outer skin layer 13 and the inner coat layer 14. Therefore,even if a high frequency surgical instrument is inserted through thebiopsy channel, there are no possibilities of electrical conductiontaking place through the metallic parts of the sheathing tube or to themanipulating head assembly 1 of the endoscope, which might lead toaccidents due to electric shocks to the operator who grips amanipulating control means on the head assembly 1.

Referring now to FIGS. 8 and 9, there is shown a flexible sheathing tube30, a third embodiment of the invention, the flexible sheathing tube 30including, from its inner side, inner and outer open helical coilmembers 31a and 31b, a protective wire mesh sleeve 32, and an outer skinlayer 33. In this case, the inner and outer helical coil members 31a and31 are overlapped one on the other, not directly but through anintermediate or inner wire mesh sleeve 34. An inner coat layer 35 iscoated in such a way as to fill in all the interstices between the innerhelical coil member 31a and the intermediate mesh sleeve 34 which isfitted on the inner coil member 31a. In this instance, the mesh sleeve34 is of metal wire netting similarly to the protective mesh sleeve 32,and likewise has no restrictions in particular with regard to the kindof metal, wire gage, mesh size, netting pattern etc.

In order to fabricate this flexible sheathing tube 30, firstly a metalstrip of a predetermined width is wound into the shape of an openhelical coil of a predetermined open pitch by the use of a rod-like jig,to form an inner open helical coil member 31a. The wire mesh sleeve 34is then fitted on the helices of the inner coil member 31a.

Thereafter, a pre-assembly of the inner coil member 31a and the wiremesh sleeve 34 is dipped in a solution of a resilient material to forman inner coat layer 35 which covers the pre-assembly all over and fillsin all the interstices between the inner coil member 31a and the wiremesh sleeve 34, followed by a drying treatment to set the inner coatlayer 35. After drying, a metal strip of a predetermined width is woundaround the wire mesh sleeve 34, similarly into the shape of an openhelical coil of a predetermined open pitch to form an outer helical coilmember 31b around the mesh sleeve 34. Nextly, a protective mesh sleeve32 and an outer skin layer 33 are successively laminated on the outerhelical coil member 31b to form the flexible sheathing tube 30.

The sheathing tube 30 of the above-described construction has bendingflexibility in all directions. When the sheathing tube 30 is bent, thegap spaces between the respective helices of the inner and outer coilmembers 31a and 31b are broadened or narrowed depending upon the degreeof bending flexures of the sheathing tube 30. Similarly, the openings inthe netting pattern of the wire mesh sleeve 34 are elongated orcontracted in relation with bending flexures of the sheathing tube 30.Particularly, the outer helical coil member 31b which is woundseparately on the outer side of the intermediate mesh sleeve 34 can bebent with a greater degree of freedom On the other hand, although theinner coil member 31a is completely embedded in the resilient materialof the inner coat layer 35, it can still be bent in a desired directionbecause of elasticity of the resilient inner coat layer 35 which fillsin even the gap spaces between the individual helices of the inner coilmember 31a. Accordingly, likewise the sheathing tube 30 as a whole hasno particular directionability in its bending flexibility.

Further, in this case, the outer coil member 31b is held in abuttingengagement not with the inner coil member 31a but with the inner coatlayer 35, which can produce greater frictional force to suppress axialpositional deviations of the outer helical coil member 31b to a minimum,preventing the pitch of helices of the outer coil member 31b from beingdisturbed in certain localities by repeated bending flexures, whichmight cause irregular variations in rigidity in the axial direction ofthe flexible rod section 2a. Of course, the inner helical coil member31a, which is wrapped in the wire mesh sleeve 34 and completely set inthe inner coat layer 35 together with the wire mesh sleeve 34, is freefrom the problem of pitch disturbances of its helices which are retainedin predetermined spaced relations by elastic restoring force of theinner coat layer 35.

The flexible sheathing tube 30 has excellent strength in axial directionas well as in radial direction. Although the intermediate wire meshsleeve 34 is in the form of a tubular netting structure which is elasticby itself, it is sandwiched between the inner and outer coil members 31aand 31b of rigid metallic material and at the same time embedded in theinner coat layer 35 substantially integrally with the inner helical coilmember 31a, in a restricted state for movements in stretching andcontracting directions, so that it contributes to improve the strengthof the sheathing tube 30 in these directions. Accordingly, in variousapplications, the flexible sheathing tube 30 is free from elongation orcontraction as caused by exertion of an axial tensile or compressiveforce, as well as from collapsing damages as caused by exertion of alarge radial compressive force. Because of the freedom from elongationor contraction in the axial direction, the flexible sheathing tube 30 isless likely to suffer from detachment of the outer skin layer 33 whichwould otherwise be caused by strong stress occurring between theprotective mesh sleeve 32 and the outer skin layer 33. Besides, theflexible sheathing tube 30 has sufficient anti-collapsing strength forprotection of the internally fitted component parts of the insertionrod.

As gathered from the foregoing description, in addition to thesatisfactory flexibility in bending directions, the flexible sheathingtube 30 has excellent properties in strengths against forces acting inaxial and radial directions. These properties of the flexible sheathingtube 30 are particularly suitable for the flexible rod section 2a of theendoscopic insertion rod 2 or the like. It can be suitably used also forthe flexible universal cable 3 or the like which contains elongatedfragile component parts.

Especially in case the sheathing tube 30 is used for the flexible rodsection 2 of the endoscopic insertion rod 2, it is required to have,along with suitable flexibility in bending directions, a certain degreeof rigidity for transmitting a propelling thrust securely as far as therigid tip end section 2c at the distal end of the insertion rod againstresistive forces which will act on the rod at the time of intracorporealinsertion. Particularly, an insertion rod for a large intestineendoscope or the like, which is expected to be met by extremely largeresistive forces, should have a higher degree of rigidity. Besides, itis desirable for the flexible rod section 2b of the insertion rod 2 tohave flexibility or rigidity varying in the axial direction thereof.Namely, it is preferred to have higher rigidity at its proximal end,which is connected to the manipulating head assembly 1 of the endoscope,from the standpoint of efficient transmission of thrust or propellingforces as explained above. On the other hand, in order to be able tofollow turning movements of the angle section 2b of the insertion rod 2to a certain degree, the flexible rod section 2a is preferred to havehigher flexibility in its fore end portion which is connected to theangle section 2b. More specifically, the flexible rod section 2a ispreferred to have higher flexibility over a length of some tenscentimeters from its fore distal end which is joined with the anglesection 2b.

Alternatively, in case the flexible sheathing tube is applied to theuniversal cable 3, it is preferred to have higher rigidity at itsopposite connecting ends, i.e., at a fore end to be connected to themanipulating head assembly 1 and a rear or proximal end with a lightguide connector to be disconnectibly connected to a light source,thereby preventing breakage of internally fitted components as caused byacute bending force on end portions of the universal cable 3, whileretaining higher flexibility in intermediate portions of the cable 3.

As will be understood from the foregoing description, it is an utmostimportance to control the bending flexibility of the sheathing tube 30in the axial direction thereof, depending upon the purpose of use. Inthis particular embodiment having the inner coil member 31a assembledwith the intermediate mesh sleeve 34 and embedded in the inner coatlayer 35 to form an integrated inner layer, axially varyingcharacteristics in bending flexibility can be easily imparted to thesheathing tube 30 by way of the just-mentioned integrated inner layer.

More specifically, since the inner coat layer 35 is formed to fill inall the gap spaces between the individual helices of the inner coilmember 31a as well as all the openings in the inner wire mesh sleeve 34,integrally embedding the inner coil member 31a and the inner mesh sleeve34 into the body of the inner coat layer 35, the resistance to bendingforces of the sheathing tube 30 can be varied by adjusting theelasticity of the inner coat layer 35. Namely, the characteristics inbending flexibility can be adjusted by changing the properties of theresilient material to be coated on. For example, as a coating resilientmaterial, there may be employed rubber material such asfluorine-contained rubber, silicon rubber or the like, or syntheticresin material such as urethane resin or the like. Of these resilientmaterials just mentioned, urethane resin has the lowest flexibility. Thedegree of flexibility can also be adjusted by varying the thickness ofcoating. The thickness of the inner coat layer 35 can be controlled byway of the kind or mixing ratio of the solvent or by way of the lengthof dipping time in the solution of the resilient material. It followsthat, by the use of these methods, the degree of bending flexibility canbe controlled accurately in the axial direction in an extremelyfacilitated manner.

It is also possible to vary the degree of bending flexibility along thelength of the sheathing tube 30 by varying the number of times ofdip-coating in the axial direction of the sheathing tube, oralternatively by varying the mesh size of the inner mesh sleeve 34 or bycoarsening its netting structure toward the fore end portion of thesheathing tube 30. The bending flexibility can be adjusted moreaccurately in a predetermined pattern by changing the number of times ofdip-coating in the axial direction of a mesh sleeve with different meshsizes or varying netting structure between fore and rear end portions ofthe sheathing tube 30.

Of course, the sheathing tube construction just described alsocontributes to improve the functions of protecting and electricallyinsulating elongated component parts which are fitted internally of thesheathing tube. Besides, in this embodiment, it is also possible to coata lubricative material on the surfaces of the inner coat layer 35 toensure friction-free sliding contact between the internally fittedcomponent parts and the inner surface of the flexible sheathing tube 30,for the purpose of protecting the internally fitted components in asecurer manner. Further, the above-described sheathing tube constructioncontributes to maintain the interior of the insertion rod in aliquid-tight sealed state, precluding possibilities of internally fittedcomponents being contaminated or deteriorated by contact with a cleaningand/or disinfectant liquid which might otherwise creep into the rodthrough a bruise or other damaged portion of the outer skin layer 33.

Referring now to FIG. 10, there is shown a flexible sheathing tube 40 asa fourth embodiment of the invention, which includes, similarly to theforegoing third embodiment, inner and outer helical coil members 41a and41b, a protective mesh sleeve 42, and an outer skin layer 43. An innermesh sleeve 44, which is fitted between the inner and outer helical coilmembers 41a and 41b, is integrally embedded in an inner coat layer 45together with the inner coil member 41a.

In this embodiment, however, the inner coat layer 45 holds not only theinner mesh sleeve 44, which is fitted on the inner helical coil member41a, but also the outer coil member 41b which is fitted around the innermesh sleeve 44.

Accordingly, the material of the inner coat layer 45 fills in openingsin the inner mesh sleeve 44 as well as gap spaces between the individualhelices of the inner and outer coil members 41a and 41b. The inner coatlayer 45 of this sort can be formed by a method similar to the ones asdescribed hereinbefore in connection with the first and secondembodiments.

The flexible sheathing tube 40 of the construction just described isimproved in strength in the axial direction because of the inner coatlayer 45 which fills up to the gap spaces between the open helices ofthe outer coil member 41b. Of course, the inner coat layer 45 which iscapable of elastic deformations has no possibilities of impairing theflexibility in bending directions. Besides, the inner coat layer 45fills in the gap spaces between open helices of the inner and outer coilmembers 41a and 41b through the inner mesh sleeve 44, precludingrelative positional deviations which would otherwise occur to the innerand outer coil members 41a and 41b when the flexible sheathing tube 40is bent. In addition to the resiliency to keep the inner and outerhelical coil members 41a and 41b from positional deviations relative toeach other, the inner coat layer 45 is preferred to be of a low-frictionmaterial such as fluorine-contained rubber or the like for the purposeof ensuring smooth sliding movements of internally fitted componentparts without additionally applying a lubricative coating on the innercoat layer.

Further, in order to impart more distinctive variations in rigidity inthe axial direction of the flexible rod section 2a, there may beemployed a flexible sheathing tube 50 with a flexible base structurewhich has inner and outer open helical coil members 51a and 51b as shownin FIG. 11. In this embodiment, of the two coil members, the outer coilmember 51b is composed of a couple of helically coiled metal strips ofdifferent widths, namely, a broad coil section 5lbB occupying almost theentire length of the flexible rod section 2a from its proximal end, anda narrow coil section 5lbN occupying a fore end portion of the flexiblerod section 2a. Broader and narrower metal strips of the broad coilsection 51bB and the narrow coil section 51bN of the outer helical coilmember 51b are butt-joined at their meeting ends, and secured to theinner helical coil member 51a by spot welding at a plural number ofspots as indicated by the letter W in FIG. 12. Indicated at 52 in FIG.11 is a protective mesh sleeve, at 53 is an outer skin layer, and at 54is an inner coat layer.

In this case, since the outer one of the helical coil members 51a and51b consists of a narrow coil section 51bN in its fore end portion asdescribed above, the sheathing tube 30 enjoys higher flexibility in itsfore end portion as compared with the rest of the sheathing tube, whichhas higher rigidity because of the increased width of the metal strip inthe broad coil section 51bB on the side of the proximal end of thesheathing tube. The greater the difference in width of the coiled metalstrip between the narrow coil section 51bN and the broad coil section5lbB, the greater becomes the difference in flexibility. Accordingly,when applied to the insertion rod 2, the flexible sheathing tube 30 ofthis construction can ensure sufficient rigidity in its proximal endportion on the side of the manipulating head assembly 1 for transmittinga propelling thrust toward the fore end of the rod upon introductioninto an intracavitary portion of patient, along with sufficient bendingflexibility in its fore end portion for following flexing movements ofthe angle section 2b of the rod.

In this regard, as shown in FIG. 12, because of the difference in widthof the joining ends of the broad and narrow coil sections 51bB and 51bNof the outer coil member 51b, the meeting end of the broad coil section51bB is narrowed down or tapered off by oblique side cuts S1 and S2 intoa width which is substantially same as or slightly broader than thewidth of the meeting end of the narrow coil section 51bN. In thisinstance, the joining end of the broad coil section 51bB shouldpreferably be narrowed down to a greater degree by the oblique side cutS1 on the side of the narrow coil section 51bN than by the side cut S2on the opposite side. By doing so, the front end face F of the broadcoil section 51bB is located in a slightly offset position to the rightin the drawing relative to the center line intermediate of the width ofits coil strip. This is because the helices of the broad coil section51bB are spaced from each other by a gap space P1 which is broader thana gap space P2 between the helices of the narrow coil section 51bN, andthe helices in the narrow coil section 51bN are disposed at a largerangle with the axis of the flexible coil structure 51. Namely, in thisinstance, arrangements are made such that an adjacently located helix ofthe narrow coil section 51bN first approaches and comes into abutmentagainst the broad coil section 51bB at a point T which coincides withthe upper end of the larger side cut S1.

In the case of the flexible coil structure of the above construction, agap space slightly broader than the gap space P1 between the helices ofthe broad coil section 51bB is formed on the right side of the splicedends of the broad and narrow coil strips, in an imbalanced relation withthe width of a gap space on the other side of the spliced ends. However,since the fore end face F of the joining end of the broad coil section51bB is joined with the narrow coil strip at a slightly offset positiontoward the side away from the narrow coil section 51bN, which is in turnlocated as close to the tapered joining end of the broad coil section51bB as possible, the flexible coil structure 51 as a whole exhibits asmooth change in flexibility or rigidity across the spliced ends, thusprecluding an abrupt change which would be normally experienced withspliced ends involving a gap space greater than P1 on one side thereof.In this instance, the width P1 of the gap spaces between the helices ofthe broad coil section 51bB are set at a suitable value which is smallenough for preventing the protective mesh sleeve 52 from sinking downbetween the respective helices into a corrugated form under pressure ofa synthetic resin material in an extrusion-molding stage for the outerskin layer 53. Accordingly, the outer skin layer 53 can be formed in auniform thickness on the protective mesh sleeve 52.

The broad coil section 51bB which is tapered into a smaller width towardthe end face F by the above-described oblique side cuts is less likelyto recoil at the spliced end. In this regard, as shown particularly inFIG. 13, recoiling at the joined end can be prevented by spot-weldingthe joining end portion to the inner helical coil member 51a at a pluralnumber of spots along the oblique side cuts S1 and S2 as indicated byletters W. Further, wasteful spaces can be minimized by rounding of fthe edges of the oblique side cuts S1 and S2 by the use of a file or thelike.

In consideration of a relatively large difference in bending flexibilitybetween the narrow coil section 51bN and the broad coil section 51bB, itmay be conceivable that, when the flexible sheathing tube 30 is bentabruptly at the spliced portion of the outer coil member 51b, the narrowcoil section 51bN could be caused to slide along with the inner coilmember 51a and to ride over the broad coil section 51bB. However, onsuch an occasion, the narrow coil section 51bN which is substantiallyheld in abutting engagement with the broad coil section 51bB at thepoint T is caused to rock on the point T at the upper end of the angularside cut S1 to increase the area of abutting engagement with theadjacent helix of the broad coil section 51bB. Accordingly, axialmovements of the narrow coil section 51bN at the abutting point aresuitably suppressed to a restricted range, preventing same from ridingover the broad coil section 51bB and exerting such an excessively largeforce on the protective mesh sleeve as would cause defoliation of theouter skin layer 53.

What is claimed is:
 1. A method for fabricating a multi-layered flexibletube for an insertion tube of an intracavitary examination instrument,said method comprising the steps of:helically winding a metal striphaving a width to form an open helical coil tube including helices in anopen pitch; fitting a protective mesh sleeve around an outer peripheryof said open helical coil tube; forming an outer skin layer to cover anouter periphery of said mesh sleeve so as to obtain a multi-layeredflexible tube structure; closing one longitudinal end of saidmulti-layered flexible tube structure with a closing member; filling aliquid resilient material through the other longitudinal end of saidmulti-layered flexible tube structure into said multi-layered flexibletube structure which is closed at the one longitudinal end; holding saidliquid resilient material in said multi-layered flexible tube structureso as to form a resilient material coat layer on an entire innerperiphery of said multi-layered flexible tube structure and so as tofill in gap spaces between open helices of said open helical coil tube;and removing said closing member to drain said liquid resilient materialfrom said multi-layered flexible tube structure.
 2. A method forfabricating a multi-layered flexible tube as defined in claim 1, whereinsaid filling step comprises providing said liquid resilient materialhaving a drying speed sufficiently low that the thickness of the innercoat can be controlled by the flow of the resilient material, furthercomprising a step of:after said removing step, retaining saidmulti-layered flexible tube structure in a position in which alongitudinal axis of said multi-layered flexible tube structure issubstantially parallel to a vertical direction in order to vary athickness of said resilient material coat layer along the longitudinalaxis.
 3. A method for fabricating a multi-layered flexible tube for aninsertion tube of an intracavitary examination instrument, said methodcomprising the steps of:helically winding a metal strip having a widthto form an open helical coil tube including helices in an open pitch;fitting a protective mesh sleeve around an outer periphery of said openhelical coil tube; forming an outer skin layer to cover an outerperiphery of said mesh sleeve so as to obtain a multi-layered flexibletube structure having opposite first and second ends in a longitudinaldirection of said multi-layered flexible tube structure; closing saidfirst end of said multi-layered flexible tube structure with a closingmember; filling a first liquid resilient material into saidmulti-layered flexible tube structure up to a first level through saidsecond end of said multi-layered flexible tube structure; holding saidfirst liquid resilient material in said multi-layered flexible tubestructure so as to form a first resilient material coat layer on a partof an inner peripheral surface of said multi-layered flexible tubestructure; removing said closing member to drain said first liquidresilient material from said multi-layered flexible tube structure;closing said second end of said multi-layered flexible tube structurewith a closing member; filling a second liquid resilient material havinghardness properties different from those of said first liquid resilientmaterial into said multi-layered flexible tube structure up to a levelpartially overlapping with an edge of said first resilient material coatlayer through said first end of said multi-layered flexible tubestructure; holding said second liquid resilient material in saidmulti-layered flexible tube structure so as to form a second resilientmaterial coat layer on the inner peripheral surface of saidmulti-layered flexible tube structure; and removing said closing memberto drain said second liquid resilient material from said multi-layeredflexible tube structure.
 4. A method for fabricating a multi-layeredflexible tube for an insertion tube of an intracavitary examinationinstrument, said method comprising the steps of:helically winding afirst metal strip having a width to form a first layer of an openhelical coil tube including helices in an open pitch; fitting a firstprotective mesh sleeve around an outer periphery of said open helicalcoil tube to form a laminated tube structure; dipping said laminatedtube structure in a liquid resilient material to form a resilientmaterial coat layer on entire inner and outer peripheries of saidlaminated tube structure; helically winding a second metal strip havinga predetermined width around an outer periphery of said coated laminatedtube structure to form a second layer of an open helical coil tube in apredetermined open pitch; fitting a second protective mesh sleeve aroundan outer periphery of said second layer of an open helical coil tube;and forming an outer skin layer to cover an outer periphery of saidsecond mesh sleeve.